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Infrared spectroscopy has been proved to be a powerful tool to study protein conformation and dynamics and, therefore, to characterize the structural properties of intrinsically disordered proteins (IDPs) and their induced folding in different environmental conditions. In this chapter, we present a general survey of the standard experimental methods to obtain the infrared absorption spectrum of a protein. The procedures required to identify the protein absorption components in the amide I region (1700 - 1600 cm - 1 ) and to assign them to the secondary structures are discussed, together with the data analysis that enable to evaluate the secondary structure content. Interestingly, this spectroscopy allows to examine proteins in different environmental conditions, both in solution and in solid form as protein films. We illustrate the potential of infrared spectroscopy on selected studies of IDP - induced folding by different effectors, such as DNA, partner proteins, and osmolytes. IDPs undergoing amyloid aggregation, as a - synuclein and a prion peptide, are also reported. © 2010 John Wiley and Sons, Inc.